In timestep 
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1) What is the norm used to evaluate the amount of divergence
left after the projection step ? 

"norm"  is  ( 1/h^2 * Div . (vector surfer_velocity))

norm is  h^2*Norm_L_2 (w3) = Norm ( Div . (vector surfer_velocity))

but surfer_velocity = dt * dimensionless_velocity / h

so norm is  Norm (dt/h * Div . (vector dimensionless_velocity))

2) Why chose this nom ? This is the fraction of volume of the celle
gained or lost in one time step. This is also the fraction 
of the volume in the whole domain lost in one time step. 
It may be necessary to multiply it by T/dt to get the volume
gained or lost in one characteristic time. 

In pres_solver.alg
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res(p)  is  (- 1/h^2 * ( (1/rho) Div . (Grad p )  )  + rhs ) 

where Div . (vector u )  = Dx u + Dy v + Dz w

and Dx u = u_{i+1/2} - u_{i-1/2}

Grad u = (Dx p, Dy p, Dz p )

norm is h^2*findn2s(res)